I. Field of the Invention
The present invention relates to communications. More particularly, the present invention relates to a novel and improved method for attaining accurate timing at a mobile station that can be used in executing multimode handoffs in a cellular system.
II. Description of the Related Art
Dual mode cellular communication systems are capable of operating in both a standard analog mode and a digital mode. Within such a system, a need may arise to handoff from analog mode to digital mode during an active call. A prolonged discontinuity in service during the handoff from analog mode to digital mode is unacceptable to the users of the system. A quality system must therefore incorporate a dual mode handoff method that causes a minimum interruption of service. However in standard analog mode, time alignment of a mobile station to a predetermined system time is not required and no analogous function is currently available. In digital mode, time alignment is critical and required for communication between the mobile station and the base station. This dissimilarity between the two modes presents a difficulty in compatibility and can cause delays in handoff from analog mode to digital mode.
Within the current analog cellular phone system, a handoff of a mobile station call can be performed between base stations during an active call. One handoff criteria which initiates a handoff is the strength of the mobile station signal arriving at the base station. Initiation of a handoff by the base station begins when the base station with which a mobile station is communicating detects a receive signal strength from the mobile station below a handoff threshold. The base station sends a request to a system controller or mobile switching center (MSC) for a handoff of the call. The system controller polls the neighboring base stations to ascertain an estimate of the signal strength of the mobile station at neighboring base stations, and, from this information, determine a candidate base station. The original base station informs the mobile station of a new frequency assignment and parameters of the candidate base station and directs the mobile station to handoff. The call is then transferred from the original base station to the candidate base station which becomes the new active base station. It should be noted that the procedure above might vary depending on the manufacturer of the equipment performing the functions.
The handoff of a mobile station call in a digital system, such as a Code Division Multiple Access (CDMA) system or Time Division Multiple Access (TDMA), is initiated either by a base station or by a mobile station. In a typical CDMA digital system, neighboring base stations use the same frequency for transmission and therefore, a mobile station can monitor signal strengths of neighboring base stations during an active call. A mobile station sends a message when one of the neighboring base stations' signal arrives at the mobile station at a level exceeding some predetermined threshold thereby initiating a handoff procedure. The system controller commands the neighboring base station to establish communication with the mobile station if resources are available at the neighboring base station. The system controller relays a message to the mobile station, through the active base station, directing the mobile station to establish communication with the neighboring base station. The mobile station establishes communication with the neighboring base station without breaking the call with the active base station. The mobile station can maintain the call with two base stations until one of the base stations signal strength falls below a predetermined level. Further information on handoffs within a digital CDMA system is disclosed in a U.S. Pat. No. 5,101,501 entitled "SOFT HANDOFF IN A CDMA CELLULAR TELEPHONE SYSTEM" issued Mar. 31, 1992, and assigned to the Assignee of the present invention. Also, further information on handoffs within a digital CDMA system is disclosed in a co-pending U.S. patent application Ser. No. 07/847,148, entitled "MOBILE STATION ASSISTED SOFT HANDOFF IN A CDMA CELLULAR COMMUNICATION SYSTEM," filed Mar. 5, 1992, now U.S. Pat. No. 5,267,261, issued Nov. 30, 1993, and assigned to the Assignee of the present invention.
A handoff from digital mode to analog mode is typically easier than the handoff from analog mode to digital mode. A digital mode to analog mode handoff could be initiated by either a mobile station or a base station on the basis of the mobile station having exited the coverage area of the digital system. The digital mode base station can transmit the necessary information about the analog system such as frequency assignment, and supervisory audio tones (SAT) and voice mobile attenuation code (VMAC) information as dictated by the system controller. Then similar to handoff within an analog system, the call can be transferred from the digital mode base station to an analog mode base station.
As a digital system is initially deployed covering only certain localities within an operating system, a need may arise to perform a handoff from analog mode to digital mode. For example, a digital system may be deployed in a metropolitan city but not deployed in the surrounding suburban areas. A user may initiate an analog call as he leaves the suburban area driving towards the city. As he approaches the metropolitan area, the heavily laden analog system may not have the capacity to accept the transfer of his call and may necessitate that the call be "dropped" thereby terminating the call before completion. If the dual mode system were capable of transferring the call from the analog system to the higher capacity digital system, the dropped call could be avoided.
Another example of a need for a handoff from analog mode to digital mode is in conjunction with the use of private branch exchange (PBX). A digital PBX system integrated into an office building might be surrounded by an analog system outside of the building. A handoff from the outside analog system to the inside digital system would take place as the user entered the building while maintaining an active call. If the system could not perform such a handoff, the call could be dropped due to the inability of the analog signals to penetrate the walls of the building and maintain signal levels capable of supporting continued communication.
A complication to the handoff from analog mode to digital mode is that digital mode requires a finite time duration for the mobile station to synchronize (acquire time alignment). The acquisition time is highly variable depending heavily on many factors including the system architecture of the digital system deployed, the signal to noise level at the mobile station at the time of attempted acquisition, the accuracy of the initial guess, and the acquisition algorithm used. In a typical CDMA system, the average system acquisition time is several seconds. The acquisition time in TDMA systems is generally less than in CDMA systems. Regardless of the case, the discontinuity of service to the end user may be unacceptable in a voice system. The present invention seeks to minimize the acquisition time by performing a preliminary time alignment of the mobile station prior to entering digital mode.
It is therefore an object of the present invention to provide in a cellular communication system a method of mobile station time alignment in analog mode for operation in digital mode.
It is another object of the present invention to provide the alignment function with minimal changes to the mobile station.
It is yet another object of the present invention to provide a alignment method which reduces acquisition time of a mobile station upon entering digital mode.